The present invention relates to an exhaust gas purification system that purifies particulate matters (PM) from the exhaust gas discharged by diesel and other internal combustion engines using a continuous regeneration-type diesel particulate filter (DPF) and also to a control method thereof.
In the same way as for NOx, CO, and also HC etc., restrictions on the volume of particulate matters (hereinafter “PM”) discharged from internal combustion engines such as diesel engines grow severe every year. Techniques for collecting this PM in a filter known as a diesel particulate filter (hereinafter “DPF”) and for reducing the quantity thereof by discharging externally have been developed.
DPFs for collecting this PM include a monolithic honeycomb form wall flow type filter made of ceramic, a fiber form type filter made of fiber shape ceramic or metal, and so on. An exhaust gas purification system using these DPFs is installed on the way of the exhaust passage of an internal combustion engine, similarly to the other exhaust gas purification systems. In addition, the exhaust gas purification system cleans and discharges exhaust gas generated in the internal combustion engine.
These DPF devices include a continuous regeneration-type DPF device wherein an oxidation catalyst is installed upstream of the DPF, a continuous regeneration-type DPF device wherein the PM combustion temperature is lowered by the effect of a catalyst supported on a filter with catalyst and PM is burned by the exhaust gas, etc.
The continuous regeneration-type DPF device wherein the oxidation catalyst is installed upstream of the DPF uses the fact that the oxidation of PM by NO2 (nitrogen dioxide) is executed at a lower temperature than the temperature in which PM is oxidized with oxygen in the exhaust gas. This DPF device is composed of an oxidation catalyst on the upstream side and a filter on the downstream side. No (nitrogen monoxide) in the exhaust gas is oxidized to NO2, by an oxidation catalyst supporting platinum or the like. PM collected by the filter on the downstream side is oxidized by this NO2 to CO2 (carbon dioxide). Thereby, PM is removed.
Besides, the continuous regeneration-type DPF device of filter with catalyst is composed of a filter with catalyst such as cerium oxide (CeO2). In this DPF device, PM is oxidized by a reaction (4CeO2+C→2Ce2O3+CO2, 2Ce2O3+O2→4CeO2, etc.) using O2 (oxygen) in the exhaust gas by means of the filter with catalyst, within the low and intermediate temperature range (on the order of 300° C. to 600° C). On the other hand, PM is oxidized by O2 (oxygen) in the exhaust gas, within the high temperature range (equal or higher than the order of 600° C.) which is higher than the temperature where PM is burned with O2 in the exhaust gas.
In this continuous regeneration-type DPF device of filter with catalyst, the oxidation catalyst is also installed on the upstream side. This oxide catalyst raises the exhaust gas temperature at the rear PM filter inlet, through oxidation reaction of unburned HC and CO in the exhaust gas, and stimulates oxidation and removal of PM. At the same time, this oxidation reaction prevents the emission of unburned HC and CO into the atmosphere.
Howsoever, these continuous regeneration-type DPF devices also cause the problem of exhaust pressure rise by the clogging of this filter. When the exhaust gas temperature is equal or higher than about 350° C., PM collected by this filter (DPF) is burned continuously and cleaned, and the filter regenerates itself. However, in the case of low exhaust gas temperature and in an operating condition of an internal combustion engine where the emission of NO is low, for example, in the case where the low exhaust gas temperature state such as idling operation of internal combustion engine, low load/low speed operation continues, the oxidation reaction is not stimulated as the exhaust gas temperature is low, the catalyst temperature lowers and the catalyst is not activated and, moreover, NO lacks. Consequently, the aforementioned reaction does not occur and the filter can not be regenerated through oxidation of PM. As a result, PM continues to be accumulated in the filter and the filter clogging progresses.
As a measure against this filter clogging, it has been conceived to forcibly burn and remove the collected PM by forcibly raising the exhaust gas temperature, when the amount of clogging has exceeded a predetermined amount. As for means for detecting the filter clogging, there are some methods such as a method for detecting by the differential pressure before and behind the filter, and a method for detecting through determination of the PM accumulation quantity by calculating in accordance of a map data in which the PM quantity to be collected is previously set based on the engine operation state. Besides, as means for exhaust gas temperature raising, there is a method by injection control of the injection in the cylinder, or a method by fuel control in the direct fuel injection in the exhaust pipe.
The cylinder injection control executes a multi injection (multi-stage injection) to raise the exhaust gas temperature in the case where the exhaust gas temperature is lower than the active temperature of an oxidation catalyst disposed upstream of the filter or supported on the filter. When the exhaust gas temperature is higher than the active temperature, a post injection (posterior injection) is executed and then the fuel in the exhaust gas is burned through the oxidation catalyst. This burning raises the exhaust gas temperature equal to or higher than a temperature at which the PM collected in the filter can be burned. As a result the filter is regenerated through burning and removing the collected PM.
Normally with continuous regeneration-type DPF devices, when the accumulated quantity of PM reaches a preset PM accumulation limit value, the operation condition of the internal combustion engine is automatically changed to forced regeneration mode. In this forced regeneration mode operation, the exhaust gas temperature is forcibly raised and the quantity of NOx is increased. And thereby, the collected PM is oxidized and removed from the filter to regenerate the filter.
Furthermore, for example, Japanese patent application Kokai publication No. 2003-155914 and Japanese patent application Kokai publication No. 2003-155916 propose to install, in the vicinity of the driver's seat, an operating means for arbitrarily actuating a forced regeneration control means so that when a large quantity of PM has accumulated in the DPF by some reasons, the driver can intentionally and immediately carry out the forced regeneration of the particulate filter. More specifically, this operating means is comprised of a warning light that indicates excessive collecting condition and a regeneration button so that the forced regeneration control can be arbitrarily actuated. Furthermore, it is also proposed to indicate a warning to carry out a human-initiated forced regeneration in which the collected PM are forcibly burned and removed when it is determined that the particulate filter has become clogged upon a confirmation of an abnormal increase in back pressure based on the differential pressure before and behind the particulate filter.
Moreover, for instance, as shown in Japanese patent application Kokai publication No. 2003-3829, an exhaust gas purification device of internal combustion engine, wherein the collected PM quantity is compared with a first predetermined quantity and a second predetermined quantity, the exhaust gas is heated for a predetermined time and regeneration acceleration through intake/exhaust throttling is executed for accelerating the oxidation removal of PM when it is more than the first predetermined quantity and less than the second predetermined quantity, forced regeneration through post-injection for forcibly burning and removing PM is executed when it is more than the second predetermined quantity, and, when the collected PM quantity is still more than the first predetermined quantity after the predetermined time, has been proposed.
Moreover, for instance, as shown in Japanese patent application Kokai publication No. 2003-3833, a regeneration control method of continuous regeneration-type DPF device for removing PM efficiently, by determining the DPF clogging state by classifying it into 3 or more clogging levels, and performing a predetermined regeneration mode operation set in correspondence to this attained clogging level when the filter clogging state attains a predetermined clogging level, has been proposed.
However, if the forced regeneration processing is executed during the traveling of a vehicle, as the engine speed is higher than that of the idling regeneration, the post-injection quantity increases necessarily, making the post-injection control during the transition phase difficult. In short, it is difficult to avoid useless injections (useless shot) such as persistent post-injection when the load changes and the engine temperature rises in the transition state, or deceleration before the exhaust gas temperature rises even if the post-injection is executed. As a result, oil dilution, which is a dilution of oil with fuel, becomes frequent. Consequently, it is not preferable that the forced regeneration processing be executed frequently. On the other hand, it has been found that it is not the case for the forced regeneration control in the idling in the vehicle stop state, and oil dilution is relatively few. Therefore, it can be conceived not to perform the forced regeneration control while the vehicle is traveling, but to perform the forced regeneration control after having stopped the vehicle.
As one of them, a method for informing the driver of the necessity of forced regeneration by means of lamp or the like when the filter is clogged to a predetermined quantity, and regenerating the filter through the execution of the forced regeneration control, by operating a manual regeneration switch provided at the driver's seat after this informed driver has stopped the vehicle, can be devised.
In order to solve the problem of oil dilution, the forced regeneration is executed by raising the exhaust gas temperature through execution of a post-injection in the cylinder of an injection quantity lower than the load in a vehicle traveling state, when the operation conditions, such as idling during this stopping, are stable. Thereby, oil dilution is limited less than the case of regeneration control in the vehicle traveling state. It should be appreciated that, if left, this oil dilution provokes problems such as abrasion or seizure of a slide portion of the engine, so the resolution thereof is important.
However, on the other hand, a vehicle has various traveling patterns and, for instance, the vehicle traveling often on a speedway has many chances to be driven at a high speed high load and the self-regeneration is accelerated without requiring the forced regeneration control as the exhaust gas temperature is high. Consequently, there is a problem that PM (collected matter) does not accumulate at the center of the filter (DPF), but is biased to accumulate outside circumferentially, causing its clogging. In short, a clogging condition which is not indicated by the differential pressure appears. If this PM biased accumulation exists, this partially accumulated PM burns almost simultaneously and the combustion enlarges suddenly, when the PM burning starts, during the forced regeneration to be executed after this biased accumulation. This enlargement causes fusion damage due to the thermal runaway producing a high temperature state in the filter. Therefore, it is also necessary to prevent this damage.
Hence, concerning the oil dilution, it has been devised to perform the forced regeneration through the execution of multi-injection and/or post-injection in the case of low exhaust gas temperature, even during the traveling, when the travel distance has exceeded a predetermined quantity, along with the forced regeneration by a manual switch, based on a finding that fuel mixed into the oil evaporates when a considerable travel distance is attained, reducing the problem of oil dilution.
However, if this method is adopted, manual regeneration will be required often, in the case of a vehicle or the like having many patterns of traveling in a low speed, high load operation state. Consequently, the interval of asking the driver to press the manual regeneration switch becomes shorter, and the lamp prompting the manual regeneration lights up more frequently. As a result, there is a problem that the driver will feel it troublesome.